==> Channel.java <==
// Demo for simulation and state management "evap"
// By RPFutrelle 4/3/01 for COM1204

/**
 * Class Channel in evap <br>
 * Shows state management in a simulation
 *
 * When message is inserted, channel goes into countdown
 * state and then nulls out ("evaporates") message.
 * Stepped by the Sim object.
 *
 * @author RP Futrelle
 * @version 1.0
 */
public class Channel {

    public static void Main(String[] args) {
        // empty for 

    }

    /** 
     * Contents of the channel.
     */
    String message;

    /**
     * State says counting down.
     */
    boolean countingDown;

    /** 
     * Countdown var, evaporate message when it == 0.
     */
    int downCount;

    /** 
     * Default constructor inits to empty channel.
     */
    void Channel() {
	countingDown = false;
	message = null;
    }

    /**
     * Accepts string and places in message; starts countdown.
     * Important to note that no counting occurs until step()
     * is called -- the key to object autonomy in our simulation.
     */
    void insertMessage(String str) {
	message = str;
	countingDown = true;
	downCount = 3;
	System.out.println("\nMessage \"" + str + "\" inserted.");
    }

    /**
     * As with all objects, responds to step().
     */
    void step() {
	if (countingDown){
	    if (downCount != 0) { // counting but not done
		downCount--;
	    }
	    else { // down to zero, so evaporate message and switch states 
		countingDown = false;
		message = null;
		System.out.println("\nPooof!  --  message evaporated");
	    }
	}
	else { // not counting, so do nothing -- idle
	}
    } // step() for Channel

} // class Channel

	    

==> Sim.java <==
// Demo for simulation and state management "evap"
// By RPFutrelle 4/3/01 for COM1204

/**
 * Class Sim in evap <br>
 * Shows state management in a simulation
 *
 * Creates the User and Channel objects
 * and steps them in a loop.
 *
 * @author RP Futrelle
 * @version 1.0
 */
public class Sim {

    public static void main(String[] args) {
        Channel chan = new Channel();
	User user = new User();
	user.attachToChannel(chan);

	for(int i = 0; i < 25; i++){
	    user.step();
	    chan.step();
	    System.out.print(i + " ");
	}

    }
} // class Sim
	    
/* Output of the above run:

Message "hi" inserted.
0 1 2 
Pooof!  --  message evaporated
3 4 5 6 
Message "hi" inserted.
7 8 9 
Pooof!  --  message evaporated
10 11 12 13 
Message "hi" inserted.
14 15 16 
Pooof!  --  message evaporated
17 18 19 20 
Message "hi" inserted.
21 22 23 
Pooof!  --  message evaporated

*/

==> User.java <==
// Demo for simulation and state management "evap"
// By RPFutrelle 4/3/01 for COM1204

/**
 * Class User in evap <br>
 * Shows state management in a simulation
 *
 * User inserts message into channel and then goes into
 * wait state.  Channel then goes into brief wait state
 * then "evaporates" message.  Then user wakes up again
 * and drops in another message.  All this is stepped
 * by the Sim object.
 *
 * @author RP Futrelle
 * @version 1.0
 */
public class User {

    public static void Main(String[] args) {
        // empty for now
    }

    /**
     * The channel attached to this User.
     */
    Channel myChannel;

    /**
     * State says counting down.
     */
    boolean countingDown;

    /** 
     * Countdown var, switch to insertion when it == 0.
     */
    int downCount;

    /** 
     * Default constructor inits to insertion state.
     */
    void User() {
	countingDown = false;
    }

    /**
     * Attaches user to channel.<br>
     * (could have used constructor argument for this)
     */
    void attachToChannel(Channel ch) {
	myChannel = ch;
    }

    /**
     * As with all objects, responds to step().
     */
    void step() {
	if (countingDown){
	    if (downCount != 0) { // counting but not done
		downCount--;
	    }
	    else { // down to zero, so switch states
		countingDown = false;
	    }
	}
	else { // not counting, so insert message and start count
	    myChannel.insertMessage("hi");
	    countingDown = true;
	    downCount = 5;
	}
    } // step() for User

} // class User